Conventional high-density magnetic storage systems require that the magnetic heads be extremely close to the storage medium, for example within one micron, in order to ensure reliable high-density recording. Such close tolerances preclude the ability to easily remove and replace the disks, so the disks must be used in a unit as a disk pack, and only removed and replaced as a disk pack unit.
One attempt to maintain and increase such high-density storage yet relieve the close tolerance problem utilizes an optical ablative coating storage medium. In such devices, a disk having a highly polished surface is coated with an ablative material. The ablative coating is an opaque material which is removable or dissipatable by application of heat. In practice, a high-power light source such as a laser is used to burn off very small portions of the ablative coating, typically of one micron diameter, at specific places on the disk as it is rotated at high speed. Controls direct the laser beam to the correct radial and circumferential position. The removal of the coating bares the specular surface beneath. Subsequently when the disk is read the reflective portions can be read as "1"s and the non-reflective or still coated portions as "0"s. One shortcoming of this type of system is that a disk, once recorded on, can not be erased or rewritten. However, the system does have the advantage of permanent high-density storage and easily removable, replaceable storage disks. It is especially advantageous for permanent or archival information storage.
The laser is operated so that it is on all the time, and a lens directs the continuous light from the laser through fiberoptics to an optical switch, which passes a sharp radiation pulse to write onto the ablative disk. One shortcoming is that the duty cycle is only about 20% of the time so that the laser is on for 80% of the time without any value to the system. Since the laser output limits and life are limited by the average output power, this wastes 80% of the laser effectiveness because it is not being used except for the 20% of the time that the electro-optical device is operated. Attempts to use switching circuits to control the power to the laser have proved unsuccessful because of the inductive delay introduced by the lines interconnecting the switching circuit and the laser.